1. Field of the Invention
This invention relates to elastic articles and a method for their production. More particularly this invention relates to ethylene-based plastomer articles having improved unload power and a process for their production. Even more particularly this invention relates to a plastomer precursor form which, by a post polymerization process of orienting and annealing, is made into an article having improved unload power as compared to the precursor article.
2. Brief Description of the Prior Art
Elastomeric polymers are utilized in a wide variety of applications for which nonelastic polymers are totally unsuited. For example, in articles of clothing, such elastomeric polymers are utilized as neck, wrist, waist, ankle and head bands. As the garment is worn, the elastic polymer band must have enough "unload power" to hold it in place. When the garment is not worn, it is generally desirable that the elastic polymer band have a low "residual set" so that the elastic polymer band of the garment returns essentially to its original shape. In addition, the elastic polymer band must also demonstrate a high degree of repeatability as the garment is worn over a long period of time. In terms of garment applications, these constraints dictate which of those elastomeric polymer compositions now known can be fabricated into article forms which can be placed into applications in garments.
"Unload power" is an important elastic tensile property in elastics applications, particularly for garment applications. For example, in diaper applications, the unload power of an elastomeric article provides an indication of the retractive force which holds the elastomeric article which is part of the diaper leg of the garment against the infant's body. In all elastomeric materials, the unload power is lower than the load power (the force required to extend the strip). This difference shows up as a hysteresis (i.e. the force to extend is different than the force to hold in place) and is larger for synthetic elastomers than in the case of a natural rubber.
"Residual set" refers to the change between the length of an elastomeric material before and after its extension to a certain length for a certain time for a certain number of cycles. For example, the percent change in length of a film after extension of the film to 200 percent of its initial length through 5 cycles. Each cycle would consist of extending the film to 200 percent of its initial length, holding the film extended for a time period, releasing the extending force, and allowing the film to return for a time period.
Typical elastic materials utilized for clothing applications include polyurethanes, ethylene-propylene rubbers (EP or EPR), including ethylene-propylene-diene terpolymers (EPDM), and natural rubbers.
Polyurethanes have the desired unload power, residual set and repeatability for use in most garment applications. However, polyurethanes have a relatively high specific gravity, which results in a lower yield of polyurethane articles hence a higher cost as compared to comparable lower specific gravity polymers. Furthermore, where the garment is to be used once and quickly discarded, such as with surgical garments or disposable diapers, polyurethanes as an element thereof are overengineered for the desired use of that garment and thus overly expensive.
For garment applications, conventional EPs and EPDMs have very poor intrinsic physical properties and for such applications generally must be blended with a plastic material such as low density polyethylene, linear low density polyethylene or ethylene vinyl acetate copolymers. Ideally, for suitability in such garment applications an EP and/or EPDM which does not need such blending has been desired.
For polyurethanes, EPs and EPDMs, to get an "accordion" shape or "gather" formation, such as around the leg opening of a disposable diaper, it has been necessary to expose that portion of the overall garment article to heat to cause shrinkage of the elastomeric article of the garment. Conventionally, used elastic materials require a relatively high shrinkage temperature. In addition, many of them require prestretching for good gather formation. Exposing the garment article to such a relatively high temperature may be detrimental to the overall properties of the garment. Further, the commonly used elastic materials generally have to be melt glued to achieve bonding to the garment, such as to a polyolefin layer in the garment. Controlled heat bonding to such a polyolefin substrate, if possible, would be advantageous.
A need exists for an elastic article that can be economically utilized in disposable garment applications. There also exists a need for use in garment applications for an elastic article with a relatively low shrinkage temperature. Additionally, there exists a need for an elastic article that can be heat bonded/sealed to polyolefins.
A need still exists for elastic articles of optimum quality for particular uses. It is still a desire of the art to provide elastic articles having high quality characteristics composed of ethylene based plastomers.
Accordingly, the present invention relates to elastic materials having improved unload powers, wherein the elastic materials comprise ethylene based plastomers made from metallocene catalysts. Such elastic materials having improved unload powers are produced by subjecting a precursor elastic film to orienting and annealing.